1. Field of the Invention
The invention relates to a power unit for a vehicle, and more particularly, to a power unit that discharges electrical charge from a smoothing capacitor in the event of a collision of a vehicle.
2. Description of the Related Art
In recent years, vehicles such as hybrid vehicles, fuel-cell-powered vehicles and the like, each having a power train in which an engine or a fuel cell or the like combined with an electric motor, have been developed and put into practical use. In such hybrid vehicles, the direct-current power of a battery is converted into alternating-current power by an inverter and then supplied to an alternating-current motor to drive the vehicle. Further, when the charge amount of the battery has decreased, the alternating-current motor is driven by an engine, and alternating-current power generated by the alternating-current motor (hereinafter referred to as a motor-generator) is converted to direct-current power by the inverter to charge the battery.
When the vehicle is driven using the motor-generator, the efficiency of driving the vehicle increases as the voltage of the battery increases. Therefore, the vehicle is equipped with a high-voltage circuit, such as a converter or the like, to raise the voltage of direct-current power supplied from the battery to a high-voltage direct-current power, and the high-voltage direct-current power is supplied to the inverter. It should be noted that such a high-voltage circuit may include a smoothing capacitor to smooth the direct-current power or reduce noise and the like, and hence includes a mechanism for discharging electrical charges (a high-voltage power) remaining on the smoothing capacitor or the like upon stoppage of the operation of the vehicle or in an emergency.
For example, Japanese Patent Application Publication No. 2006-224772 (JP-A-2006-224772) describes a power unit for a vehicle that is equipped with discharge means for discharging high-voltage power remaining in a high-voltage circuit and on a smoothing capacitor or the like after the supply of a power from a battery is stopped upon detection of a collision of the vehicle. The power unit interrupts the supply of direct-current power from the battery to the high-voltage circuit by means of a system main relay if a collision of the vehicle is detected, and turns on a discharge relay that connects the smoothing capacitor in series with a resistor to discharge electrical charge remaining in the smoothing capacitor. The power unit ensures safety about the high-voltage power through this operation.
However, in the described power unit, the control of the system main relay and the control of the discharge relay are required in the event of a collision. It is therefore required that a control system survive to discharge electrical charge from the smoothing capacitor even during and after a collision through an improvement in the collision survivability performance of the power unit. In Japanese Patent Application Publication No. 2009-90818 (JP-A-2009-90818) an art for enhancing the survivability of the control system is described. Specifically, JP-A-2009-90818 describes an art for reducing the damage to on-vehicle components such as an inverter and the like by restraining the on-vehicle components from colliding with other members or the like in the event of a collision of a vehicle, to improve the collision survivability performance of a power unit.
In the related art, there are four methods of ensuring safety about the high voltage of a vehicle, namely, (A) the protection against direct contact with a high-voltage portion, (B) the protection against indirect contact with the high-voltage portion, (C) the reduction in voltage/energy of the high-voltage portion in the event of breakage of the power unit, and (D) the securement of an insulating resistance through the mounting of the high-voltage portion at such a position as to prevent breakage in the event of a collision. In this case, as regards the methods (A) and (B), safety is ensured by providing an insulating cover for the high-voltage portion, an interlock mechanism, or the like. Further, as regards the method (C), there is a measure based on collision detection and a discharge treatment as described in JP-A-2006-224772. As regards the method (D), there is an improvement in anti-collision performance or the like as described in JP-A-2009-90818.
However, in the power unit described in JP-A-2006-224772 it is required that a collision be detected, and that the control system survive even during and after the collision to perform the treatment of discharging electrical charge from the smoothing capacitor. Thus, for example, it is necessary to double the circuit or improve the collision survivability performance of the circuit. However, an increase in the cost of the circuit may be entailed. Further, as an alternative to the idea of doubling the circuit or the like, there is also a method of completing discharge instantaneously in the event of a collision. However, a large discharge resistor is needed to terminate the discharge of a high-voltage power in a short time.
Furthermore, there is also a method of reinforcing a mechanical structure of a control system of a power unit to prevent the control system from being damaged and a method of restraining on-vehicle components from colliding with other members or the like as disclosed in Japanese Patent Application Publication No. 2009-90818 (JP-A-2009-90818). However, in addition to the difficulty in reducing the size of the components, an increase in cost and a deterioration in vehicle mountability may be caused.